Regional distribution of calretinin and calbindin-D28k expression in the brain of the urodele amphibian Pleurodeles waltl during embryonic and larval development

Molecular identity of the lateral lemniscus nuclei in the adult mouse brain

Introduction

The dorsal (DLL), intermediate (ILL), and ventral (VLL) lateral lemniscus nuclei are relay centers in the central auditory pathway of the brainstem, commonly referred to as the lateral lemniscus nuclei (LLN). The LLN are situated in the prepontine and pontine hindbrain, from rhombomeres 1 to 4, extending from the more rostral DLL to the caudal VLL, with the ILL lying in between. These nuclei can be distinguished morphologically and by topological and connectivity criteria, and here, we set out to further characterize the molecular nature of each LLN.

Methods

We searched in situ hybridization studies in the Allen Mouse Brain Atlas for genes differentially expressed along the rostrocaudal axis of the brainstem, identifying 36 genes from diverse functional families expressed in the LLN.

Results

Available information in the databases indicated that 7 of these 36 genes are either associated with or potentially related to hearing disorders.

Discussion

In conclusion, the LLN are characterized by specific molecular profiles that reflect their rostrocaudal organization into the three constituent nuclei. This molecular regionalization may be involved in the etiology of some hearing disorders, in accordance with previous functional studies of these genes.

Cell-type profiling in salamanders identifies innovations in vertebrate forebrain evolution

The evolution of advanced cognition in vertebrates is associated with two independent innovations in the forebrain: the six-layered neocortex in mammals and the dorsal ventricular ridge (DVR) in sauropsids (reptiles and birds). How these innovations arose in vertebrate ancestors remains unclear. To reconstruct forebrain evolution in tetrapods, we built a cell-type atlas of the telencephalon of the salamander Pleurodeles waltl. Our molecular, developmental, and connectivity data indicate that parts of the sauropsid DVR trace back to tetrapod ancestors. By contrast, the salamander dorsal pallium is devoid of cellular and molecular characteristics of the mammalian neocortex yet shares similarities with the entorhinal cortex and subiculum. Our findings chart the series of innovations that resulted in the emergence of the mammalian six-layered neocortex and the sauropsid DVR.

Identifying Amygdala-Like Territories in Scyliorhinus canicula (Chondrichthyan): Evidence for a Pallial Amygdala

To identify the putative amygdalar complex in cartilaginous fishes, our first step was to obtain evidence that supports the existence of a pallial amygdala in the catshark Scyliorhinus canicula, at present the prevailing chondrichthyan model in comparative neurobiology and developmental biology. To this end, we analyzed the organization of the lateral walls of the telencephalic hemispheres of adults, juveniles, and early prehatching embryos by immunohistochemistry against tyrosine hydroxylase (TH), somatostatin (SOM), Pax6, serotonin (5HT), substance P (SP), and Met-enkephalin (MetEnk), calbindin-28k (CB), and calretinin (CR), and by in situ hybridization against regulatory genes such as Tbr1, Lhx9, Emx1, and Dlx2. Our data were integrated with those available from the literature related to the secondary olfactory projections in this shark species. We have characterized two possible amygdalar territories. One, which may represent a ventropallial component, was identified by its chemical signature (moderate density of Pax6-ir cells, scarce TH-ir and SOM-ir cells, and absence of CR-ir and CB-ir cells) and gene expressions (Tbr1 and Lhx9 expressions in an Emx1 negative domain, as the ventral pallium of amniotes). It is perhaps comparable to the lateral amygdala of amphibians and the pallial amygdala of teleosts. The second was a territory related to the pallial-subpallial boundary with abundant Pax6-ir and CR-ir cells, and 5HT-ir, SP-ir, and MetEnk-ir fibers capping dorsally the area superficialis basalis. This olfactory-related region at the neighborhood of the pallial-subpallial boundary may represent a subpallial amygdala subdivision that possibly contains migrated cells of ventropallial origin.

Regional chemoarchitecture of the brain of lungfishes based on calbindin D-28K and calretinin immunohistochemistry

Lungfishes are the closest living relatives of land vertebrates, and their neuroanatomical organization is particularly relevant for deducing the neural traits that have been conserved, modified, or lost with the transition from fishes to land vertebrates. The immunohistochemical localization of calbindin (CB) and calretinin (CR) provides a powerful method for discerning segregated neuronal populations, fiber tracts, and neuropils and is here applied to the brains of Neoceratodus and Protopterus, representing the two extant orders of lungfishes. The results showed abundant cells containing these proteins in pallial and subpallial telencephalic regions, with particular distinct distribution in the basal ganglia, amygdaloid complex, and septum. Similarly, the distribution of CB and CR containing cells supports the division of the hypothalamus of lungfishes into neuromeric regions, as in tetrapods. The dense concentrations of CB and CR positive cells and fibers highlight the extent of the thalamus. As in other vertebrates, the optic tectum is characterized by numerous CB positive cells and fibers and smaller numbers of CR cells. The so-called cerebellar nucleus contains abundant CB and CR cells with long ascending axons, which raises the possibility that it could be homologized to the secondary gustatory nucleus of other vertebrates. The corpus of the cerebellum is devoid of CB and CR and cells positive for both proteins are found in the cerebellar auricles and the octavolateralis nuclei. Comparison with other vertebrates reveals that lungfishes share most of their features of calcium binding protein distribution with amphibians, particularly with salamanders.

Comparative Analysis of Nkx2.1 and Islet-1 Expression in Urodele Amphibians and Lungfishes Highlights the Pattern of Forebrain Organization in Early Tetrapods

Expression patterns of Nkx2.1 and Islet-1 (Isl1), which encode transcription factors that are key in the regionalization of the forebrain, were analyzed by combined immunohistochemical methods in young adult specimens of two lungfishes (Neoceratodus forsteri and Protopterus dolloi) and a urodele amphibian (Pleurodeles waltl). We aimed to get insights into the possible organization of the forebrain in the common ancestor of all tetrapods because of the pivotal phylogenetic significance of these two groups, being lungfishes the closest living relatives of tetrapods, and representing urodeles a model of simple brain organization with most shared features with amniotes. These transcription factors display regionally restricted expression domains in adult (juvenile) brains that are best interpreted according to the current prosomeric model. The regional patterns observed serve to identify regions and compare between the three species studied, and with previous data reported mainly for amniotes. We corroborate that Nkx2.1 and Isl1 expressions have very similar topologies in the forebrain. Common features in all sarcopterygians (lungfishes and tetrapods) have been observed, such as the Isl1 expression in most striatal neurons, whereas Nkx2.1 is restricted to migrated interneurons that reach the ventral pallium (VP). In the pallidal derivatives, the combination of both markers allows the identification of the boundaries between the ventral septum, the bed nucleus of the stria terminalis (BST) and the preoptic commissural region. In addition, the high Isl1 expression in the central amygdala (CeA), its boundary with the lateral amygdala (LA), and the scattered Nkx2.1 expression in the medial amygdala (MeA) are also shared features. The alar and basal hypothalamic territories, and the prethalamus and posterior tubercle (TP) in the diencephalon, have maintained a common pattern of expression. This regional distribution of Isl1 and Nkx2.1 observed in the forebrain of urodeles and lungfishes contributes further to our understanding of the first terrestrial vertebrates and their ancestors.

Homeostatic and regenerative neurogenesis in salamanders

Large-scale regeneration in the adult central nervous system is a unique capacity of salamanders among tetrapods. Salamanders can replace neuronal populations, repair damaged nerve fibers and restore tissue architecture in retina, brain and spinal cord, leading to functional recovery. The underlying mechanisms have long been difficult to study due to the paucity of available genomic tools. Recent technological progress, such as genome sequencing, transgenesis and genome editing provide new momentum for systematic interrogation of regenerative processes in the salamander central nervous system. Understanding central nervous system regeneration also entails designing the appropriate molecular, cellular, and behavioral assays. Here we outline the organization of salamander brain structures. With special focus on ependymoglial cells, we integrate cellular and molecular processes of neurogenesis during developmental and adult homeostasis as well as in various injury models. Wherever possible, we correlate developmental and regenerative neurogenesis to the acquisition and recovery of behaviors. Throughout the review we place the findings into an evolutionary context for inter-species comparisons.

Husbandry of Spanish ribbed newts (Pleurodeles waltl)

Research on urodele amphibians, such as newts, is constantly contributing to our understanding of fundamental biological processes. In the present chapter, we present detailed husbandry protocols for the Spanish ribbed newt (Pleurodeles waltl ). We describe the main phases of their life cycle, with emphasis on the progressive development of sensory, motor, and integration systems, which lead to the acquisition of specific stereotyped (and conditioned) behaviors. The methods are outlined to manage housing, feeding, handling, captive breeding, health monitoring, and euthanasia in this species under laboratory conditions. With minor changes, these protocols can also be applied to other species of urodele amphibians commonly used in laboratory research.

Regional expression of Pax7 in the brain of Xenopus laevis during embryonic and larval development

Pax7 is a member of the highly conserved Pax gene family that is expressed in restricted zones of the central nervous system (CNS) during development, being involved in early brain regionalization and the maintenance of the regional identity. Using sensitive immunohistochemical techniques we have analyzed the spatiotemporal pattern of Pax7 expression in the brain of the anuran amphibian Xenopus laevis, during development. Pax7 expression was first detected in early embryos in the basal plate of prosomere 3, roof and alar plates of prosomere 1 and mesencephalon, and the alar plate of rhombomere 1. As development proceeded, Pax7 cells were observed in the hypothalamus close to the catecholaminergic population of the mammillary region. In the diencephalon, Pax7 was intensely expressed in a portion of the basal plate of prosomere 3, in the roof plate and in scattered cells of the thalamus in prosomere 2, throughout the roof of prosomere 1, and in the commissural and juxtacommissural domains of the pretectum. In the mesencephalon, Pax7 cells were localized in the optic tectum and, to a lesser extent, in the torus semicircularis. The rostral portion of the alar part of rhombomere 1, including the ventricular layer of the cerebellum, expressed Pax7 and, gradually, some of these dorsal cells were observed to populate ventrally the interpeduncular nucleus and the isthmus (rhombomere 0). Additionally, Pax7 positive cells were found in the ventricular zone of the ventral part of the alar plate along the rhombencephalon and the spinal cord. The findings show that the strongly conserved features of Pax7 expression through development shared by amniote vertebrates are also present in the anamniote amphibians as a common characteristic of the brain organization of tetrapods.